1. Field of the Invention
The present invention relates generally to a screw-driven linear actuator having an inner cylinder mounted for projection and retraction with respect to an outer cylinder so as to hoist or lower heavy things. More particularly, this invention relates to a linear actuator equipped with a mechanism for preventing an inner cylinder from dropping when a driven nut, which is threaded with a drive screw shaft for linear reciprocating movement of the inner cylinder, is damaged or otherwise broken.
2. Description of the Related Art
Conventionally, screw-driven linear actuators having an inner cylinder provided for projection and retraction with respect to an outer cylinder are widely used for hoisting or lowering heavy things. The outer cylinder is fixed to a housing, and the inner cylinder is telescopically connected to the outer cylinder so that the inner cylinder can move to project from and retract within the outer cylinder while a load, such as a heavy thing, is supported by the inner cylinder.
The housing is provided with a drive screw shaft driven for rotation in both forward and reverse directions at a fixed position by means of a rotating drive source such as a motor. The drive screw shaft is threaded with a driven nut fixed to the inner cylinder so that upon rotation of the drive screw shaft, the inner cylinder is moved to project from or retract within the outer cylinder.
The drive screw shaft is made of steel, and the driven nut threaded with the drive screw shaft is made of brass casing which is softer than steel. The conventional linear actuator of the foregoing construction may encounter a problem that the driven nut is broken due to fatigue caused by continuous use under loaded condition, or the driven nut becomes disengaged from the drive screw nut due to wearing-out of an internal screw thread of the driven nut.
Another problem is that when the inner cylinder is subjected to an overload which is in excess of an allowable maximum load of the linear actuator, the driven nut may be damaged or otherwise broken, allowing the inner cylinder to drop abruptly. If such a sudden dropping of the inner cylinder occurs during lifting or lowering operation of the heavy loads, it will result in a serious accident.